Disposal of future VOCs and chlorinated volatile organic compounds (CVOCs), as well as cleanup of past spills which have found their way into soil and groundwater, requires a major long-term worldwide commitment. Current methods for detoxifying soil and groundwater (i.e., soil washing, retorting, pump and treat, etc.) are slow, inefficient and very costly, yet no viable in-situ or other technology has emerged to replace the prior art technology. Additionally, even after the removal of the VOCs or CVOCs, these toxic contaminants must be either concentrated or recycled, or converted into benign products prior to their release.
Current methods for recycling or conversion typically involve carbon adsorption followed by steam stripping and VOC collection or destruction using thermal or catalytic means. Illustrative of this type of collection apparatus is shown in U.S. Pat. No. 5,069,689 to Goldhast, where VOC fumes are adsorbed onto activated carbon, desorbed and oxidized in a combustion zone in the presence of oxygen. There is no mention of the ability to catalytically destroy VOCs, and the high temperatures employed in the process (500.degree.-1,000.degree. C.) certainly are not indicative of a catalytic process.
U.S. Pat. No. 4,966,611 to Schumacher et al., also describes a typical activated carbon bed which functions in two cycles: an adsorption cycle and a regeneration cycle. The process requires that a gas stream contact an adsorbent material during adsorption, that the adsorbent material be heated by a humidified gas during regeneration, and that the resulting compounds desorbed from the heated adsorbent material be burned in a combustion unit during regeneration. The combustion is not catalytic, and uses outside fuel such as natural gas.
Another approach is illustrated in U.S. Pat. No. 4,780,287 to Zeff et al. The VOC stream is passed through a porous bed of silica or quartz chips, and the bed is thereafter irradiated with ultraviolet light to effect a photolytic decomposition. In the event that the decomposition rate declines, the coking of the absorbent can be reversed through heating and/or passing nitrogen gas through the adsorbent bed.
Ying et al., U.S. Pat. No. 4,623,464 illustrates a combination of physicochemical and biological treatment processes, including the use of powdered activated carbon in an enhanced sequencing batch reactor. And Grantham et al., U.S. Pat. No. 4,526,677, uses activated carbon as the adsorbent, although other less active, and less selective adsorbents such as coconut charcoal, petroleum coke or devolatized and activated coal may also be used. Polymeric carbonaceous materials were also believed to be effective.
However, these prior art approaches are energy and equipment intensive, requiring multiple adsorption beds, the capability for feed switching and steam stripping, as well as facilities for continuous heating of large gas volumes. It has been estimated that groundwater treatment plants could save 30-40% in traditional capital and operating costs if separate parallel carbon beds and associated CVOC conversion facilities and services were made unnecessary.
What has been needed in the prior art is an a medium which is capable of acting both as a sorbent for VOCs and/or CVOCs, and as a catalyst for the subsequent oxidative destruction of such adsorbed material with a catalyst which in a preferred embodiment would include an exchanged and impregnated zeolite-based catalytic medium which is uniquely active, selective and stable during VOC and/or CVOC oxidation. One of the key features would be the ability to act as a molecular sieve, pulling out large quantities of volatiles at ambient temperature. With periodic increases in temperature, the collected CVOCs would not only desorb from the catalyst bed, but also simultaneously be catalytically oxidized, thereby eliminating the need for carbon beds.